This invention relates to target detection systems. More particularly it relates to a target detection system comprising means for repetitively deriving picture signals representing an image of a scene, which picture signals may contain a target signal to be detected against background signals, means for calculating and storing the values of a set of features of an accumulated set of background picture signals and for calculating and storing the statistical distribution of each feature from the accumulated set, means for storing predetermined values of the same set of features of a second set of picture signals due to known targets and for calculating and storing the statistical distribution of each feature from the second set, means for comparing the statistical distribution of the background features with that of the target features to set a target decision threshold as a function of feature values, and means for comparing the feature values of an incoming picture signal with the threshold to detect a target.
Such target detection systems are used in imaging infrared target detection systems where the expected target is of limited spatial extent, typically only a few picture elements (pixels) in width and height. A typical target might be an approaching sea-skimmming missile, infrared radiation being generated by the propulsion motor of the missile or by the heated nose cone of the missile in the case of a supersonic missile.
Such target detection systems assume that the image is homogeneous, whereas in fact most images contain several distinct regions. For example a seascape image will contain an area of sky, several distinct areas in the sea as well as a horizon area. Additionally there is a variation in background conditions as a function of azimuth due to solar radiation and to cloudscapes. Thus the set of background picture signals and the features derived from them may have so large a distribution that the setting of a single threshold may lead to targets being overlooked at some elevations or azimuths, or to an excessive false alarm rate at others.